Though poly(vinyl chloride) is a polymer of wide commercial use, it is known to have inadequate rigidity or heat distortion temperature above 75.degree. C. To overcome this deficiency in PVC, alloying polymers of higher heat distortion temperatures than PVC have been blended with PVC to produce binary alloys having a HDT intermediate between that of PVC and that of the alloying polymer. Such alloying polymers, for example an imidized poly(methyl methacrylate) (HT510 from Rohm and Haas), can form a miscible blend with PVC, an alloy which is characterized by a single phase having a single glass transition temperature (Tg) intermediate between that of the PVC and that of the alloying polymer. Other alloying polymers, for example, form partially miscible blends with PVC, an alloy in which case two phases occur, a lower Tg phase having a major portion of the PVC and a minor portion of alloying polymer, and a higher Tg phase having a major portion of alloying polymer and minor portion of PVC.
Alloying polymers that are effective in forming miscible or partially miscible blends with PVC have generally little or no chlorine in their chemical structures so they are flammable, in contrast to PVC, which is known to be resistant to burning and which is often used in applications where good flame resistance is needed. Thus, blending an alloying polymer with PVC decreases PVC flame resistance. Furthermore, because most alloying polymers tend to be more brittle than PVC, their addition to PVC causes an increase in brittleness of the PVC binary alloy. This usually requires adding more impact modifier to maintain adequate impact strength in the PVC alloy. Since one of the main purposes of making PVC alloys is to increase HDT, it is therefore commercially desirable to overcome the deficiencies which usually accompany the formation of such binary alloys.
Recent disclosures illustrate the use of alloying polymers with PVC or CPVC in binary blends. U.S. Pat. No. 4,255,322 discloses blends containing vinyl chloride polymers and copolymers with polyglutarimides with the weight ratio of said polymers of 5:95 to 95:5. These compatible blends are claimed to have improved heat distortion temperatures and are impact modifiable. The vinyl chloride polymers can be poly(vinyl chloride), chlorinated poly(vinyl chloride) or poly(vinyl chloridevinyl acetate).
U.S. Pat. No. 4,745,156 discloses blends composed of 55-75 weight percent poly(vinyl chloride) polymers and 10-30 weight percent .alpha.-methyl styrene/acrylonitrile copolymer. The remainder may include impact modifiers. The said poly(vinyl chloride) polymer may include 5-50 parts chlorinated poly(vinyl chloride) having chlorine levels between 60 and 70%. The materials are said to possess improved vacuum and pressure forming properties. The said compositions have improved heat distortion temperature, high flame retardancy and chemical resistance when vinyl chloride polymer component is about 55 weight percent.
U.S. Pat. No. 4,847,331 discloses a CPVC compound comprising a blend of CPVC having a chlorine content of between about 60 percent and 66 percent by weight and a poly(methyl methacrylate) comprising not more than 80 percent by weight of the blend. The composition of matter displays homogeneous, substantially single phase behavior with the CPVC and poly(methyl methacrylate) being substantially or completely miscible. The resulting blended material exhibits enhanced temperature and durability properties and remains homogeneous up to a temperature of 230.degree. C.
U.S. Pat. No. 4,647,626 discloses a blend of CPVC having chlorine content between 60 percent and 67 percent by weight and a styrene-acrylonitrile (SAN) copolymer having acrylonitrile content between 15 percent and 25 percent by weight. A single homogeneous phase is presumed from a single glass transition endotherm.
U.S. Pat. No. 4,710,533 discloses a CPVC blend including the aforementioned SAN copolymer and a flow enhancing polymer such as alpha-methyl styrene. Impact modifiers such as ABS are also suggested.